Posted by *Josepi* on April 8, 2010, 3:09 am

There is another term for it. "Bi-phase supply"

This is all OT as the OP asked about a two phase 120/208 supply usage...LOL

====snip====

*> Since the centre tap of a supply transformer from a utility is almost*

*> always*

*> grounded (in N.America) and used for the reference, by default the two *

*> 120*

*> volt outside legs are 180 degrees out of phase by definition.*

*> Yes this is still single phase, and two phase is an incorrect*

*> reference, but*

*> the "phases" ("legs" if you will?) are always 180 degrees out of phase ****

*> WITH RESPECT TO GROUND OR CENTRE TAP ****

*> ---------- clarified the implied reference-------------------*

I'm getting heartily sick of the confusion over what is normally

referred to as a "Bi-Phase" supply. In the UK such supplies are normally

only seen on building sites where a 55-0-55 vac supply is used to power

tools in order to limit the maximum rms voltage with respect to ground

to that of any of the two anti-phase lives (or hots) thus greatly

reducing the electrocution hazard.

The tools are rated for 110 volt operation and receive exactly this by

using the two lives (hots). Effectively the supply is derived from a

centre tapped 110v secondary where the centre tap is earthed to prevent

an earth fault that would otherwise cause the voltage on the other leg

rising to 110v if the secondary were left 'floating'.

The stateside domestic dwelling supply is simply the same scheme but

with double the voltage to feed high wattage loads and the centre tap

connected to the neutral return (which is tied to the local substation

earth). Lighting circuits and low wattage appliances make use of just

one of the bi-phase lives (hots) and the neutral.

This has the advantage of a reduced electrocution risk and, as far as

GLS tungsten filament lamps are concerned, allow a more robust filament

for any given wattage to be used compared to the UK and european

standards of 240 and 220 volts respectively[1].

Essentially, for a given wattage, a 115v lamp can use half the filament

length at twice the CSA compared to its 230v counterpart. This gives the

choice of either the same life with a higher efficiency or a longer life

at the same efficiency compared to a 230 volt lamp.

[1] Although the EC have 'Harmonised'(tm) the domestic PSU voltage

levels to a nominal "230" volt, the allowable tolerances in each country

of the EC have simply been adjusted to avoid the need to actually make

any change to the original 240 and 220 volt supplies.

Most appliances designed for a nominal 230volt supply will work

perfectly fine on either voltage but tungsten filament lamps are an

exception to this rule since they are extremely sensitive to the effects

of voltage variation on their service life and efficiency and are

therefore designed for the voltage used in the region they are marketed

in. In this regard, the modern electronically ballasted CFL has the edge

over the traditional tungsten filament GLS lamp.

--

Regards, John.

Please remove the "ohggcyht" before replying.

The address has been munged to reject Spam-bots.

Posted by *You* on April 7, 2010, 5:31 pm

*> This could go on forever, but it won't. This is really my last posting *

*> on this topic.*

*> *

*> Take a transformer secondary.*

*> *

*> It makes 240 volts.*

*> *

*> It has only ONE current flow in it.*

*> *

*> Put a tap in the winding at exactly the mid point.*

*> *

*> Measure the voltage in each half, moving the meter leads up in unison.*

*> *

*> Both readings are identical and at the same phase angle.*

*> *

*> Putting a centre tap on that winding has NOT reversed the current in *

*> half the winding. The current is still the same as it was, at the SAME *

*> angle all the way through the winding.*

The above is EXACTLY why you don't understand what is REALLY GOING ON...

If you had just spent a little time listening in EE 101, instead of

sleeping, you would know just where your assumptions above, are leading

you down a Blind Alley, and why thew rest of us are laughing at your

ignorance.....

Posted by *Josepi* on April 7, 2010, 6:40 pm

Nobody is laughing. I can see what he is saying. Not everybody has studied

electrical vestors / phasors and this is very abstract for some.

His analogy on the batteries was an excellent one to help everybody

understand. Just a small detail was missed.

Your exagerated, over the top, personal attack doesn't help him at all. We

have dealt with this behaviour from another pair of hot heads (W+G) here for

years now and it has destroyed these groups, somehwat.

Note: Single phase on the transformer refers to the primary connection.

The above is EXACTLY why you don't understand what is REALLY GOING ON...

If you had just spent a little time listening in EE 101, instead of

sleeping, you would know just where your assumptions above, are leading

you down a Blind Alley, and why thew rest of us are laughing at your

ignorance.....

Posted by *m II* on April 7, 2010, 7:32 am

Before some nit picker finds my 'mistakes',

> There is 120 degrees between the rms peaks of the sine waves.

'rms peaks' should have read 'peaks'.

> 180^2 + 103.92^2 = 43009

> square root of 14399.36 = 208 volts.

should have read:

square root of 43009 = 208 volts

mike

Posted by *m II* on April 1, 2010, 10:43 pm

Bruce in alaska wrote:

*>> If indeed they were 180 degrees apart, a meter would read zero volts *

*>> instead of 240. Think of two batteries in series, with the meter *

*>> connected to the '+' of one and the '-' on the other.*

*> *

*> Nope, you got it backwards... If they are 0 Degrees apart the meter *

*> would read Zero Volts, HOWEVER if they are 180 degrees apart the Meter *

*> would read 240 Vac. Draw it out, and you can see the Phase relationship.*

I have and I did. Many years ago.

Take two identical cars at the same speed having a head on collision.

The total vector force (mass X velocity) of both cars becomes ZERO.

That is what happens when you have a 180 degree difference between two.

Now, for electrical stuff. The proof of what I say may be found in the

common, or neutral conductor. If both hot sides are feeding identical

loads, the neutral of the circuit is carrying zero amperes.

The neutral carries only the *difference* in load currents.

If the voltage polarities were 180 apart, the neutral would carry the

SUM of the load currents. It doesn't.

Check out the Edison three wire circuit. It's applicable to batteries as

well as 240/120 volt residential distribution.

What you two are confusing is the sum of the current flows in the

neutral and the flow in the sources. The sources are IN PHASE and at 0

degrees.

===============================================

Vector Summation.—This is a simple geometrical process for ascertaining

the pressure at the free terminals of alternating current circuits. The

following laws should be carefully noted:

If two alternating pressures which **agree in phase are connected

together in series**, the voltage at the free terminals of the circuit

will be equal to their arithmetical sum, as in the case of direct currents.

http://www.meekmark.com/dp/Hawkins7/projectID421e9ab940c97.html

================================================

Please look at figs 2,125 to 2,128 and duplicate the numbers with a

ruler, pencil and protractor. The numbers don't lie.

The magnitude of the resultant voltage INCREASES as the angle of the

phase difference between two vectors DECREASES. Zero phase angle

difference gives the highest possible resultant voltage.

mike

> Since the centre tap of a supply transformer from a utility is almost> always> grounded (in N.America) and used for the reference, by default the two> 120> volt outside legs are 180 degrees out of phase by definition.> Yes this is still single phase, and two phase is an incorrect> reference, but> the "phases" ("legs" if you will?) are always 180 degrees out of phase ***> WITH RESPECT TO GROUND OR CENTRE TAP ***> ---------- clarified the implied reference-------------------